Generally, combustion engines must be initially turned over using other energy sources. During startup of reciprocating engines, the movement of the pistons or crankshaft assembly creates a compression pressure in the cylinder head space. A substantial crankshaft torque is required to produce this compression pressure to start the engine. The same principle applies to starting compressors.
External combustion machines, for example Stirling cycle machines including engines and refrigerators, have a long technical heritage. Walker, Stirling Engines, Oxford University Press (1980), describing Stirling cycle engines in detail, is incorporated herein by reference. The principle underlying the Stirling cycle engine is the mechanical realization of the Stirling thermodynamic cycle: isovolumetric heating of a gas within a cylinder, isothermal expansion of the gas (during which work is performed by driving a piston), isovolumetric cooling, and isothermal compression.
To start a Stirling engine, the working fluid in the cylinder head must be compressed to a relatively high pressure. The energy required to compress the working fluid is higher than the net thermal energy that is transformed into mechanical energy on a given cycle of the engine. Consequently the starting torque is much greater than the operational output torque of the engine. Such comparatively large demands during start-up may require the use of higher power rated starting motor and associated electronics that increase the overall energy cost to the system.
Traditionally, devices such as compression release valves have been used to reduce the required starting torque by reducing the compression pressure. Such compression release valves work by reducing the compression pressure in the cylinder head space while the engine is started, and restoring the compression pressure after the engine has started. When a compression valve is opened, some of the engine working fluid can be released from the compression chamber, thus reducing the cylinder head compression pressure and allowing the engine to turn over with less torque. When the engine starts, the valve is closed and the compression pressure is restored.
Traditional compression valves, such as electrically controlled valves with poppet construction with a solenoid actuator and a spring return, do not address the need of external combustion engines that require high working fluid flow and high bi-directional pressures. If such traditional valves are to be used in external combustion engines with high working fluid flow and high bi-directional pressures, the valves will require relatively large spring returns, and correspondingly high power demands for the efficient transfer of the fluids. Furthermore, such valves are not usually amendable to controlled precision closing required for certain applications. Accordingly, the requirements of low engine starting torque and the need for precision control of valve closing, in combination with the size and power demands of such traditional valves, make such valves impractical and inadequate for certain applications in external combustion engines.
There is therefore the need for valves that are relatively compact with minimal power requirements, have controlled precision closing, and can efficiently transfer fluids between two compartments of the external combustion engine. Furthermore, the compression valves should be capable of operating without lubrication and over a wide temperature range with relatively minimal leakage.